Marine Flora and Fauna of Hong Kong and Southern China IV

Muricid predation upon an under boulder community of epibyssate bivalves in the Cape d'Aguilar Marine Reserve, Hong Kong

The Marine Flora and Fauna ofHong Kong and Southern China IV (ed. B. Morton). Proceedings of the Eighth International Marine Biological Workshop: The Marine Flora and Fauna of Hong Kong and Southern China, Hong Kong, 2-20 April 1995. Hong Kong: Hong Kong University Press, 1997. MURICID PREDATION UPON AN UNDER-BOULDER COMMUNITY OF EPIBYSSATE BIVALVES IN THE CAPE D'AGUILAR MARINE RESERVE, HONG KONG Elizabeth Harper Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, U.K. and Brian Morton The Swire Institute of Marine Science, The University of Hong Kong, Cape d' Aguilar, Shek 0, Hong Kong ABSTRACT In Lobster Bay, within the Cape d'Aguilar Marine Reserve, there are a number of bivalve species which are byssally attached to the undersurfaces of large, stable, boulders. Isognomon legumen, Cardita leana, Hormomya mutabilis, Barbatia virescens, Striarca symmetrica and Didimacar tenebrica are the six most abundant species. They are eaten by at least three species of muricid gastropods, i.e., Morula musiva, Thais clavigera and T. luteostoma. Observations of empty valves collected from the shell gravels of Lobster Bay showed that each of the six bivalve species were victims of gastropod attack. Analysis of these shells show that for each species there was a naturally preferred boring site, indicating stereotyped predator feeding behaviour. There was, however, no positive correlation between predator size and the size of prey item selected. Laboratory experiments demonstrate that Morula musiva and Thais clavigera display a clear preference for the pleurothetic Isognomon legumen, followed by the orthothetic Hormomya mutabilis . The arcoids, Barbatia virescens and Striarca symmetrica, were less commonly consumed, and then only when the more preferred species had been depleted. Didimacar tenebrica and Cardita leana were not eaten in these experiments. Analysis of borehole positioning by individual predators gave no indication of stereotypy. Studies of shell thickness at the preferred boring site and of the dry weight flesh yields were measured for each of the potential bivalve prey taxa. They indicate that Isognomon legumen (and Hormomya mutabilis) is probably preferred because of its 264 E. HARPER & B. MORTON thinner shell. Even though the arcoids, with their plump shells, have high flesh yields, their valves are much thicker and thus more time (and energy) consuming to penetrate. Comparison of the field and laboratory observations highlight significant differences between patterns of predation in Lobster Bay and laboratory aquaria. Although in the laboratory Morula musiva and Thais clavigera feed optimally, we suggest that in the patchy and highly dynamic environment of Lobster Bay they select prey items on the basis of time rather than energy optimization. In this respect they behave in a manner similar to congenerics and other taxa of predatory muricids which have been studied on more open rocky coasts. INTRODUCTION Muricid gastropods are important predators in the shallow waters around Hong Kong and prey frequently upon other molluscan species (Taylor 1980). At Cape d'Aguilar there is a diverse fauna of intertidal muricids, the predatory behaviour of which has been studied by a number of authors (Tong 1986; Cha 1994; Harper and Morton 1994). Most of these studies have dwelt upon predation of either elements of the Septifer virgatus community on the exposed shores of the peninsula or on the shallow burrowing bivalves in Lobster Bay, which are easily exhumed by wave action and thus subjected to muricid attack. The boulder field at the southeast end of Lobster Bay, between the tip of the peninsula and the island of Kau Pei Chau, supports a diverse invertebrate fauna (Morton and Harper 1995). Among the taxa present, are a number of muricid gastropods which move over and between the boulders and a variety of bivalve species which are attached to their surfaces. Of the muricid species present, some are known to feed on bivalve prey by boring a hole in the shell and extracting the flesh through it, in the manner described by Carriker (1981). Examination of empty bivalve shells washed onto the shores of Lobster Bay revealed that large numbers had been punctured by straight-sided holes attributable to muricids (Carriker and Yochelson 1968), thereby indicating that at least some of these gastropods must be considered important predators of the bivalves living in this community. In this paper we investigate the behaviour of predatory muricids feeding on bivalves which live amongst the boulders of Lobster Bay. PREDATORY MURICIDS IN LOBSTER BAY At least seven species of muricid gastropod occur intertidally in Lobster Bay. Of these, Thais luteostoma (Holten), T. clavigera (Kuster) and Morula musiva...